Optical transmission system using an optical phase conjugation device

ABSTRACT

An optical system having an optical fiber path suitable for propagating an optical signal at least in a first direction, and a plurality M of optical amplifiers disposed along the optical fiber path so as to divide the optical fiber path in N spans of optical fiber. The spans of optical fiber all have substantially a length L amp  and have at least one transmission optical fiber having an effective length L eff . An optical phase conjugation device is associated to one of the amplifiers of the plurality of amplifiers and is disposed in combination with a dispersion compensator. The compensator is disposed upstream from the amplifier associated to the optical phase conjugation device and is adapted for introducing an accumulated dispersion such as to substantially compensate the dispersion accumulated in a portion having a length (L amp −L eff ) of the span immediately upstream from the amplifier associated to the optical phase conjugation device.

The present invention relates to an optical transmission system using anoptical phase conjugation device.

Long-distance optical transmission systems have been constructed byusing erbium-doped fiber amplifiers (EDFAs) as in-line opticalrepeaters. Signal attenuation due to fiber loss is periodicallycompensated for by the optical amplifier gain to overcome the limitationof transmission distance. Since, in such systems, signal power ismaintained at a high level along the entire system length owing to theperiodic amplification, the dependence of fiber refractive index onoptical power can no longer be ignored. This nonlinear effect, calledthe Kerr effect, leads to the self-phase modulation (SPM) of opticalpulses, which in turn interplays with the group-velocity dispersion(GVD), or chromatic dispersion, in the fiber, causing nonlinear waveformdistortion. In order to realize long-distance (e.g. 1000-2000 km ormore) signal transmission at high data transmission rate (e.g. 40 Gbit/sor more) this waveform distortion must be counteracted.

Optical phase conjugation (OPC) is a known technique for chromaticdispersion compensation. Details may be found in G. P. Agrawal,“Fiber-Optic Communication Systems” A Wiley Interscience Publication,(1997), at paragraph 9.7. As explained by Agrawal, under certainconditions, OPC can compensate simultaneously for both GVD and SPM.Pulse propagation in a lossy optical fiber is governed by the Non-LinearSchrödinger Equation (NLSE) $\begin{matrix}{{\frac{\partial A}{\partial z} + {\frac{\mathbb{i}}{2}\beta_{2}\frac{\partial^{2}A}{\partial t^{2}}}} = {{{\mathbb{i}}\overset{\_}{\gamma}{A^{2}}A} - {\frac{1}{2}\alpha\quad A}}} & \lbrack 1\rbrack\end{matrix}$where A=A(z, t) represents a slowly varying amplitude of a pulseenvelope, β₂ is the GVD coefficient of the optical fiber, related to thedispersion parameter D by the following relation $\begin{matrix}{D = {{- \frac{2\pi\quad c}{\lambda^{2}}}\beta_{2}}} & \lbrack 2\rbrack\end{matrix}${overscore (γ)} is the nonlinear coefficient of the optical fiber, i.e.governs the SPM, and α accounts for the fiber loss. When α=0 (loss lesscase), A* satisfies the same equation when one takes the complexconjugate of eq.[1] and changes z to −z. As a result, midspan OPC cancompensate for SPM and GVD simultaneously. Clearly, such case isimmaterial, as fiber losses cannot be practically avoided.

In order to study the impact of the fiber loss, the followingsubstitution may be madeA(z,t)=B(z,t)exp(−αz/2)  [3]so that eq.[1] can be written as $\begin{matrix}{{\frac{\partial B}{\partial z} + {\frac{\mathbb{i}}{2}\beta_{2}\frac{\partial^{2}B}{\partial t^{2}}}} = {{\mathbb{i}\gamma}\quad z{B}^{2}B}} & \lbrack 4\rbrack\end{matrix}$where γ(z)={overscore (γ)} exp(−ααz). By taking the complex conjugate ofeq.[4] and changing z to −z, it can be seen that perfect SPMcompensation can occur only if γ(z)=γ(L−z), where L is the total systemlength. This condition cannot be satisfied for α≠0.

One may think that the problem can be solved by amplifying the signalafter midspan OPC such that the signal power becomes equal to the inputpower before the signal is launched in the second-half section of thefiber link. Although such an approach can reduce the impact of SPM,actually it does not lead to a satisfactory compensation of the SPM.Perfect SPM compensation can occur only if the power variations aresymmetric around the midspan point where the OPC is performed so thatγ(z)=γ(L−z) in eq.[4]. In practice, signal transmission does not satisfythis property. One can come close to SPM compensation if the signal isamplified often enough that the power does not vary by a large amountduring each amplification stage. This approach is, however, notpractical since it requires closely spaced amplifiers.

S. Watanabe, in U.S. Pat. No. 6,175,435, considers a phase conjugatordisposed between a transmission line I (of length L₁) and a transmissionline II (of length L₂). After a series of calculations, he obtains thefollowing equations for GVD and SPM compensation:D ₁ L ₁ =D ₂ L ₂  [5]γ₁ {overscore (P)} ₁ L ₁=γ₂ {overscore (P)} ₂ L ₂  [6]where {overscore (P)}₁ and {overscore (P)}₂ denote the average powers inthe transmission lines I and II, respectively. Also, D₁ and γ₁ denotethe dispersion parameter and the nonlinear coefficient in thetransmission line I, respectively; and D₂ and γ₂ denote the dispersionparameter and the nonlinear coefficient in the transmission line II,respectively. According to the patent, complete compensation can berealized by providing, at positions equivalently symmetrical withrespect to the phase conjugator, the same ratio of the optical Kerreffect to the dispersion. An increase of this ratio along thetransmission line can be attained by gradually decreasing the dispersionor gradually increasing the optical Kerr effect. It is possible tochange the dispersion value by adequately designing the fiber. Forexample, the above ratio is changeable by changing the zero dispersionwavelength of a dispersion shift fiber (DSF) or by changing the relativerefractive index between the core and the clad of the fiber or the corediameter thereof. Meanwhile, change of the optical Kerr effect can beachieved by changing the nonlinear refractive index of the lightintensity. According to Watanabe, a suitable optical fiber can bemanufactured by continuously changing at least one fiber parameterselected from the loss, nonlinear refractive index, mode field diameterand dispersion.

In Applicant's opinion, the use of such kinds of “special” fibers doesnot represent an optimal solution, as such fibers may be complex tomanufacture. Further, such method does not apply to optical systemsalready installed, unless a substitution of all the fibers of the systemis performed.

C. Lorattanasane et al., in “Design Theory of Long-Distance OpticalTransmission Systems Using Midway Optical Phase Conjugation” Journal ofLightwave Technology, vol. 15, no. 6, pages 948-955 (1997), describe adesign method for suppressing the residual waveform distortion due toperiodic power variation in an optical amplifier chain and to dispersionvalue fluctuation from span to span along a midway optical phaseconjugation system. According to the authors, the amplifier spacing mustbe short relative to the nonlinearity length and signal pulses must betransmitted within appropriate windows of fiber dispersion. Computersimulation required for long-distance systems, whereas, forshort-distance systems less than 1000 km, the amplifier spacing as longas 100 km is possible.

In Applicant's opinion, an amplifier spacing as long as 100 km also forlong distance systems, having a length higher than 1000 km, ispreferred, in order to reduce the number of installed amplifiers.

WO patent application no. 99/05805, to British Telecommunications PLC,discloses a method for symmetrised mid-span spectral inversion (MSSI),where the high power regions in the optical communication system aresymmetrised about the MSSI means. The amplifiers are positioned so as tohave the high-power regions in the two sections of the transmission linksymmetrical about the mid-point of the transmission network, where MSSIis performed. These high-power regions are the length of fiberimmediately after the fiber amplifier which is substantially equal tothe effective nonlinear length (L_(eff)) of the optical transmissionlink. The distance from the amplifier preceding the phase conjugator tothe phase conjugator is L_(A) and the distance from the phase conjugatorto the subsequent amplifier is L_(B). The distances L_(A) and L_(B) aregiven by $\begin{matrix}{L_{A} = {{\frac{L_{amp} + L_{eff}}{2}\quad L_{B}} = \frac{L_{amp} - L_{eff}}{2}}} & \lbrack 7\rbrack\end{matrix}$where L_(amp)=L_(A)+L_(B) is the amplifier spacing. In an example,L_(amp) is 80 km, L_(eff) is 21.5 km, so that the MSSI equipment wouldbe sited at a distance of ≈51 km from the preceding amplifier. With anodd number of spans, if it is not possible to place the MSSI equipmentat a location other than an amplifier site, the author suggests to add alength of fiber L_(amp)−L_(eff) kilometers long immediately after theMSSI equipment at the amplifier location. Thus a length of fiber of 58.5km would be added. With an even number of spans, the MSSI equipment issited immediately upstream of the optical amplifier and a length offiber L_(eff) kilometers long is sited immediately upstream of the MSSIequipment. The author admits it may be necessary to insert additionalamplifiers to give the symmetrical positioning of the high-power regionsor if the optical signal levels are sufficiently low so as to cause biterror rate degradation.

In Applicant's opinion, a positioning of the optical phase conjugatorvery far from an amplifier (e.g. about 50 km) has a drawback in that theoptical line has to be provided with a dedicated site for the MSSIequipment, in addition to the amplifier sites. Even when lengths offibers are added as suggested in the '805 patent application in order toplace the MSSI equipment at an amplifier site, the necessity arises ofproviding additional amplifiers to take into account the long length ofthe added fiber (in particular with an odd number of spans). Suchcombination of long added fiber and additional amplifiers may, in turn,unbalance the power distribution along the line, so that nonlinearitycompensation may be hindered.

The Applicant has understood that these problems may arise due to thefact that only a symmetric dispositions in space, that is, in physicallength of fiber, has been considered in '805 patent application for thehigh power regions with respect to the position of the OPC device. TheApplicant has found that more advantageous system configurations forreducing nonlinearity exploiting an OPC device may be implemented byconsidering symmetrised dispositions of the high power regions withrespect to the dispersion accumulated along the fiber path, rather thanwith respect to the fiber path itself.

More particularly, the Applicant has found that nonlinearity may besubstantially reduced in a system comprising spans of transmissionoptical fiber separated by optical amplifiers by associating an opticalphase conjugation to an optical amplifier, advantageously in the samesite including the optical amplifier, the optical phase conjugationdevice being in combination with a dispersion compensator, suitable forintroducing an amount of accumulated dispersion such as to substantiallycompensate the dispersion accumulated in a portion of a span having alength (L_(amp)−L_(eff)), wherein L_(amp) is the average span length andL_(eff) is the effective length of the optical fibers used in the spans.For the purposes of the present invention, by “dispersion compensator”(or simply “compensator”) of a portion of a span having a length L_(x)it has to be intended a device having a length L_(comp)<L_(x), capableof introducing an amount of accumulated dispersion equal, in absolutevalue, to the accumulated dispersion introduced in the portion of thespan. Accordingly, the dispersion compensator used in the presentinvention has a length L_(comp)<(L_(amp)−L_(eff)). Preferably thedispersion compensator has a length L_(comp)<(L_(amp)−L_(eff))/3. Forexample, the compensator may be a dispersion compensating optical fiber.If the compensator introduces an amount of accumulated dispersion havingthe same sign of the dispersion accumulated along the optical fiber usedin the spans, the OPC device is placed upstream from the compensator. Ifthe compensator introduces an amount of accumulated dispersion havingopposite sign with respect to the dispersion accumulated along theoptical fiber used in the spans, the OPC device is placed downstreamfrom the compensator. The compensator may have a length much lower than(L_(amp)−L_(eff)). Reducing length and attenuation of the dispersioncompensator allows to limit the onset of nonlinear effects. Nonlineareffects can be further reduced by appropriately selecting the materialor medium used in the dispersion compensator.

In a first aspect the invention relates to an optical system comprisingan optical fiber path suitable for propagating an optical signal atleast in a first direction; a plurality M of optical amplifiers,disposed along said optical fiber path, so as to divide said opticalfiber path in N spans of optical fiber, said spans of optical fiberhaving substantially a length L_(amp) and comprising at least onetransmission optical fiber having an effective length L_(eff), and anoptical phase conjugation device associated to an amplifier of saidplurality of amplifiers. In the optical system, the optical phaseconjugation device is disposed in combination with a dispersioncompensator, said compensator being disposed upstream from saidamplifier associated to the optical phase conjugated device, saidcompensator being adapted for introducing an accumulated dispersion suchas to substantially compensate a dispersion accumulated in a portionhaving a length (L_(amp)−L_(eff)) of a span immediately upstream fromsaid amplifier associated to said optical phase conjugation device.

The dispersion compensator can have a sign of dispersion equal to orrespectively opposite with respect to a sign of dispersion of said spanimmediately upstream from said amplifier at a wavelength of said opticalsignal, and said optical phase conjugation device is disposed upstreamor respectively downstream from said dispersion compensator.

The dispersion compensator can include a length of optical fiber,preferably having an absolute value of dispersion coefficient higherthan or equal to three times the dispersion coefficient of saidtransmission optical fiber at a wavelength of the optical signal.

Alternatively, the dispersion compensator can include a chirped fibergrating.

The optical amplifiers can comprise erbium-doped fiber amplifiers.

The system can comprise a transmitting station, a receiving station,said transmitting station being connected at an input end and saidreceiving station being connected to an output end of said optical fiberpath.

In a second aspect the invention relates to a method for assembling anoptical system comprising the steps of: providing a plurality M ofoptical amplifiers; connecting said plurality of optical amplifiers by Nspans of optical fiber so as to form an optical fiber path, the spans ofoptical fiber having substantially a length L_(amp) and comprising atleast one transmission optical fiber having an effective length L_(eff),associating a phase conjugation device to an amplifier along saidoptical fiber path. The step of associating comprises: disposing acompensator upstream from said amplifier associated to the optical phaseconjugated device, and disposing said phase conjugation device incombination with said dispersion compensator, said compensator beingadapted for introducing an accumulated dispersion such as tosubstantially compensate a dispersion accumulated in a portion having alength (L_(amp)−L_(eff)) of a span immediately upstream from saidamplifier associated to said optical phase conjugation device.

In a third aspect the invention relates to a method of operating of anoptical transmission system comprising an optical fiber path comprisingat least one transmission optical fiber having an effective lengthL_(eff) and a plurality of optical amplifiers disposed along saidoptical fiber path, so as to divide said optical fiber path in N spansof optical fiber having substantially a length L_(amp). The methodcomprises: inserting an optical signal at an input end of said opticalfiber path; amplifying said optical signal along said fiber spans bysaid plurality of optical amplifiers; accumulating a dispersion of saidoptical signal along the optical fiber path; phase-conjugating saidoptical signal near a first amplifier of said plurality of opticalamplifiers, so that said optical signal diminishes its accumulateddispersion, in absolute value, after said step of phase-conjugating. Themethod further comprises passing said optical signal, before said firstamplifier, through a compensator, said compensator introducing anaccumulated dispersion such as to substantially compensate a dispersionaccumulated in a portion having a length (L_(amp)−L_(eff)) of a spanimmediately upstream from said first amplifier.

In a fourth aspect the invention relates to a method of upgrading anoptical transmission system comprising an optical fiber path, theoptical fiber path including at least one transmission optical fiberhaving an effective length L_(eff) and a plurality of optical amplifiersdisposed along the optical fiber path, so as to divide the optical fiberpath in N spans of optical fiber having substantially a length L_(amp).The method comprises disposing a phase conjugation device in associationwith one of said plurality of optical amplifiers, in combination with adispersion compensator disposed upstream from said one amplifier, saidcompensator being adapted for introducing an accumulated dispersion suchas to substantially compensate a dispersion accumulated in a portionhaving a length (L_(amp)−L_(eff)) of a span immediately upstream fromsaid one amplifier associated to said optical phase conjugation device.

Further features and advantages of the present invention will be betterillustrated by the following detailed description, herein given withreference to the enclosed drawings, in which:

FIG. 1 schematically shows an optical transmission system according tothe invention;

FIG. 2 schematically shows a power profile that can be obtained alongthe optical fiber path of the system of FIG. 1 using lumped erbium dopedfiber amplifiers;

FIGS. 3 a and 3 b show how the eye opening may worsen due to the onsetof nonlinearity in a high power transmission system;

FIG. 4 shows a portion of a first embodiment of optical system accordingto the present invention;

FIG. 5 schematically shows a plot of the optical power of an opticalsignal traveling in the portion of optical system according to FIG. 4(continuous line) and a plot of the accumulated dispersion thereof(dashed line);

FIG. 6 shows the eye opening penalty versus the length of addedcompensating fiber, obtained in a simulated system of 600 km accordingto the first embodiment of the invention;

FIGS. 7 a and 7 b schematically show plots of the optical power versusaccumulated dispersion, respectively for a system according to the priorart and for a system according to the first embodiment of the invention;

FIG. 8 shows a portion of a second embodiment of optical systemaccording to the invention;

FIG. 9 schematically shows a plot of the optical power of an opticalsignal traveling in the portion of optical system according to FIG. 8(continuous line) and a plot of the accumulated dispersion thereof(dashed line);

FIG. 10 schematically shows a plot of the optical power versusaccumulated dispersion, for a system according to the second embodimentof the invention;

FIG. 11 shows the comparative result of experiments performed by theApplicant with a configuration using an OPC and a compensator accordingto FIG. 4 (continuous thick line), with a configuration using an OPConly (continuous thin line), and with a configuration with no OPC(dashed thin line);

FIG. 12 shows the eye opening penalty versus the length of addedcompensating fiber, obtained in a simulated system corresponding to thesystem of the experiment of FIG. 11 but with a bit rate of 40 Gbit/s;

FIGS. 13 a-b show two eye diagrams of the simulation of FIG. 12;

FIG. 14 shows the eye opening penalty versus the length of addedcompensating fiber, obtained in a simulated system of 1800 km accordingto the first and second embodiment of the invention;

FIGS. 15 a-b show two eye diagrams of the simulation of FIG. 14;

FIG. 16 shows the penalty versus the amplifier number for a system of1800 km having an OPC and 20 km of an additional fiber according to thefirst embodiment of the invention.

FIG. 1 schematically shows an optical transmission system 10 accordingto the invention, comprising a transmitting station 11 a, adapted totransmit optical signals over an optical fiber path 12, and a receivingstation 11 b, adapted to receive optical signals coming from the opticalfiber path 12. The transmitting station 11 a comprises at least onetransmitter. The receiving station 11 b comprises at least one receiver.For WDM transmission, stations 11 a, 11 b comprise a plurality oftransmitters and receivers, for example twenty or thirty-two orsixty-four or one hundred transmitters and receivers. The transmissionsystem may include transmitting and receiving stations and an opticalfiber path to transmit signals in a direction opposite to that ofoptical fiber path 12. Terminal and line apparatuses operating in thetwo directions often share sites and facilities.

The transmitter or transmitters included in the transmitting station 11a provide an optical signal to be coupled into the optical fiber path12. Typically, each transmitter may comprise a laser source, adapted toemit a continuous wave optical signal having a predetermined wavelength,and an external optical modulator, for example a lithium niobatemodulator, adapted to superimpose on the continuous wave optical signalemitted by the laser source a traffic signal at a predetermined highfrequency or bit rate, such as for example 10 Gbit/s or 40 Gbit/s.Alternatively, the laser source may be directly modulated with thetraffic signal. A preferred wavelength range for the optical signalradiation is between about 1460 nm and about 1650 nm. A more preferredwavelength range for the optical signal radiation is between about 1520nm and about 1630 nm. Optical signals may be of the return-to-zero (RZ)format or non-return-to-zero (NRZ) format. Typically, in case of WDMtransmission each transmitter may also comprise a variable opticalattenuator, adapted to set a predetermined power level for each signalwavelength (pre-emphasis level). In case of WDM transmission, thedifferent signal wavelengths emitted by the plurality of transmittersare multiplexed by a suitable multiplexing device on the optical fiberpath 12. Such multiplexing device can be any kind of multiplexing device(or combination of multiplexing devices), such as a fused fiber orplanar optics coupler, a Mach-Zehnder device, an AWG (Arrayed WaveguideGrating), an interferential filter, a micro-optics filter and the like.

Each receiver is adapted to convert an incoming optical signal in anelectrical signal. Typically, this task may be provided by aphotodetector. The receiver may also extract the traffic signal from theelectrical signal. For a WDM transmission, a plurality of photodetectorsis provided. A demultiplexing device allows to separate the differentsignal wavelengths from a single optical path to a plurality of opticalpaths, each terminating with a receiver. The demultiplexing device canbe any kind of demultiplexing device (or combination of demultiplexingdevices), such as a fused fiber or planar optics coupler, a Mach-Zehnderdevice, an AWG (Arrayed Waveguide Grating), an interferential filter, amicro-optics filter and the like.

The optical fiber path 12 comprises at least one transmission opticalfiber. The transmission optical fiber used in the optical fiber path 12is a single mode fiber. For example, it can be a standard single modeoptical fiber (SMF), having chromatic dispersion between approximately+16 ps/(nm·km) and +20 ps/(nm·km) at a wavelength of 1550 nm, or adispersion-shifted fiber (DSF), having a dispersion approaching zero ata wavelength of 1550 nm, or a non-zero dispersion fiber (NZD), withdispersion of between approximately 0.5 ps/(nm·km) and 4 ps/(nm·km), inabsolute value, at a wavelength of 1550 nm, or a fiber of thehalf-dispersion-shifted type (HDS) having a positive dispersion which isintermediate between that of an NZD type fiber and a standardsingle-mode fiber. In order to reduce the occurrence of four-wave-mixing(FWM), the optical transmission fiber or fibers included in the opticalfiber path 12 may preferably have a dispersion which is greater than orequal to approximately 0.5 ps/(nm·km), more preferably greater than orequal to 1 ps/(nm·km), in absolute value, at a wavelength of 1550 nm.Preferably, if the optical signals are of the RZ format, a transmissionfiber having a chromatic dispersion higher than 15 ps/(nm·km) inabsolute value at 1550 nm may be used, for example a SMF fiber.Preferably, if the optical signals are of the NRZ format, a transmissionfiber having a negative chromatic dispersion lower than 10 ps/(nm·km) inabsolute value at 1550 nm may be used.

A plurality of M optical amplifiers is disposed along the optical fiberpath 12, so as to divide the optical fiber path 12 in a plurality offiber spans. In FIG. 1 six optical amplifiers 13 ¹, 13 ² . . . , 13 ⁶are disposed along the optical fiber path 12, so that five fiber spans14 ¹, 14 ² . . . , 14 ⁵ may be identified. Typically the opticalamplifiers are included in suitable amplification sites along theoptical path.

For example, an optical amplifier suitable to be used in the systemaccording to the present invention is an erbium doped fiber amplifier,comprising at least one pump source suitable for providing an opticalpumping radiation, at least one erbium doped fiber and at least onecoupler device suitable for coupling the pumping radiation and anoptical signal to be amplified into the erbium doped fiber or fibers,e.g. a WDM coupler. Suitable pumping radiation may preferably have awavelength in a range around 1480 nm or in a range around 980 nm.

Another exemplary optical amplifier suitable to be used in the systemaccording to the present invention is a semiconductor amplifier,comprising an electrical pump source suitable for providing electricalpower and a semiconductor optical amplifying element comprising anelectrode structure adapted for connection to the electrical pumpsource.

A further example of optical amplifier suitable to be used in a systemaccording to the present invention is a lumped Raman amplifier,comprising at least one pump source adapted for providing an opticalpumping radiation having a power and a wavelength suitable for causingRaman amplification in a piece of optical fiber especially adapted forobtaining high Raman amplification in a length of several km (Ramanfiber), typically having a low effective area, included in the opticalamplifier, and at least one coupler device suitable for coupling suchpumping radiation into the Raman fiber, e.g. a WDM coupler. In order tohave Raman amplification, the wavelength of the pumping radiation shouldbe shifted with respect to the wavelength of the signal radiation in alower wavelength region of the spectrum, such shift being equal to theRaman shift (see G. P. Agrawal, “Nonlinear Fiber Optics”, Academic PressInc. (1995), pag. 317-319) of the material comprised in the core of theRaman fiber. For typical silica/germania-based fibers the Raman shift isequal to about 13.2 THz. For signal wavelengths around 1550 nm, pumpingradiation wavelengths suitable for Raman amplification may have awavelength around 1450 nm. As an example, a fiber suitable for a lumpedRaman amplifier is disclosed in the article: T. Tsuzaki et al.,“Broadband Discrete Fiber Raman Amplifier with High Differential GainOperating Over 1.65 μm-band”, OFC2001, MA3-1.

N fiber spans 14 ¹, 14 ² . . . , 14 ^(N) are identified between thetransmitting station 11 a and the receiving station 11 b as the portionsof optical fiber path 12 lying between the M optical amplifiers 13 ¹, 13² . . . 13 ^(M). If the last optical amplifier disposed along theoptical fiber path 12 is disposed immediately upstream from thereceiving station 11 b, for setting the power of the optical signal to asuitable level before the introduction in the receiving station 11 b,the number M of optical amplifiers is higher than the number N of thespans by a unity (M=N+1). If a span of fiber is placed between the lastoptical amplifier and the receiving station 11 b, it holds M=N.Preferably, the optical fiber path 12 comprises an even number of fiberspans N.

Preferably, the length of each span is greater than or equal to 40 km,more preferably greater than or equal to 80 km. Shorter span lengths maybe provided, in particular, in long-haul systems, i.e. systems having anoverall length exceeding several thousands of km, e.g. 10.000 km, inwhich the onset of nonlinear effects may sum up along the optical fiberpath, until high levels. On the other hand, greater span lengths inexcess of 80 km are desirable for systems having an overall length of nomore than 2-3000 km, in which the onset of nonlinear effects may occurdue to an increase of the overall optical power of the signal sent onthe optical fiber path (for example due to an increase of the channelssent in a WDM system) and/or of the bit rate of the system.

Preferably, the optical amplifiers 13 ¹ . . . 13 ^(M) are disposedsubstantially periodically along the optical fiber path 12, that is, thelength of the fiber spans 14 ¹ . . . 14 ^(N) is substantially the same.Practically, this may correspond to a variation of the length of thespans in the system of at most 10%, preferably 5%, of the average lengthof the spans. More particularly, a lower variation may be desirable forsystems having, for example, overall length in excess of 1500 km, and/orusing a bit rate of 40 Gbit/s or more, and/or using a high number ofchannels.

An optical phase conjugation (OPC) device 15 is disposed along theoptical fiber path 12 near one of the optical amplifiers. The OPC device15 may be a device capable of inverting the spectrum of the channelstransmitted along the line, i.e. a device for spectral inversion.Additionally, such device may modify the central wavelength of theinverted channels. Preferably, the OPC device 15 is apolarization-independent device. Preferably, it comprises a non-linearmedium through which the optical channels and at least one linearlypolarized pumping radiation pass twice, in one direction on the firstpass and in the opposite direction on the second pass. On the secondpass, the optical channels pass through the non-linear medium afterundergoing a rotation of π/2 of their polarization state. Thepolarization state of the pumping radiation remains unchanged throughoutthe double pass. An example of a device of this type is described in thearticle by C. R. Giles, V. Mizrahi and T. Erdogan,“Polarization-Independent Phase Conjugation in a Reflective OpticalMixer”, IEEE Photonics Technology Letters, Vol. 7, No. 1, pp. 126-8(1995). Typically, the OPC device 15 can comprise one or more devicesfor filtering the residual wavelengths of the non-linear wavelengthconversion process. Additionally, the OPC device can comprise one ormore devices for amplification of the phase conjugated channels or, ingeneral, for total or partial compensation of the attenuation of thephase conjugator. Preferably, the wavelength conversion may be carriedout so as to provide phase conjugated signals having a wavelengthshifted of not more than 5 nm with respect to the wavelength of thesignals inputted in the OPC device. In order to perform the phaseconjugation of many different channels, a multi-channel OPC device ofthe type described in U.S. Pat. No. 5,365,362 may be used. Thedisposition of the OPC device near the optical amplifier will bediscussed in great detail in the following.

At the output of each optical amplifier the power of the optical signalis increased to a level determined by the optical gain provided by theamplifying medium. FIG. 2 schematically shows an optical power profilethat can be obtained along a portion of the optical fiber path 12 of thesystem of FIG. 1 with a chain of lumped amplifiers (e.g. EDFAs): theposition of the optical amplifiers is shown by the dashed verticallines. In particular, in FIG. 2 it is shown that the power increasesabruptly in a very small length, corresponding to the overall length ofthe lumped amplifier (e.g. few meters for an EDFA, few millimeters oreven less in a semiconductor amplifier, some km for a lumped Ramanamplifier), and then diminishes progressively due to the attenuationintroduced by the optical fiber included in the span downstream from theamplifier, until the next optical amplifier, in which the powerincreases abruptly another time, and so on. As schematically shown byFIG. 2 the power profiles upstream and downstream from the opticalamplifiers are clearly not symmetrical with respect to the position ofthe optical amplifiers.

The maximum level of optical power along the optical fiber path, that isthe height of the peaks in FIG. 2, depends on many factors. Typically,it depends on the optical gain introduced by the optical amplifiers:such optical gain may be for example regulated as a function of theoverall length of the system, and/or of the span lengths, and/or of thenumber of the channels in a WDM system. A system having higher bit ratemay reach higher power level along the optical fiber path with respectto a system having lower bit rate, as the available time slot for eachbit of information is lower. Today there is a great interest inincreasing the bit rate of optical systems from values of about 2.5Gbit/s or 10 Gbit/s to higher values such as 40 Gbit/s or more. Anincrease of the bit rate may cause a corresponding increase of theimpact of nonlinear effects, as the reached power levels along the linemay be very high. As an example, FIGS. 3 a and 3 b show the result oftwo simulations made by considering the launch of a single opticalchannel at 40 Gbit/s having an average power of 10 dBm in a systemhaving a length of 400 km and with perfect compensation of chromaticdispersion. In FIG. 3 a nonlinear effects were canceled by setting tozero the nonlinear coefficient of the fiber. In FIG. 3 b a nonlinearcoefficient of 1.3 1/(W·km) was introduced. As it can be seen, the eyeopening is much lower in FIG. 3 b, even in a system having a relativelylow length, due to the onset of nonlinear effects. It has to be noticedthat the value of 10 dBm of average power of the optical channel waschosen only for simulation purposes: it has to be intended that theinvention applies also to systems using lower average power signals.

In order to locate the portions of optical fiber path in which the powerlevel of the optical signal reaches high values, the effective lengthL_(eff) may be used: $\begin{matrix}{L_{eff} = \frac{1 - {\mathbb{e}}^{{- \alpha}\quad L_{amp}}}{\alpha}} & \lbrack 8\rbrack\end{matrix}$where L_(amp) is the average span length and a is the attenuationcoefficient of the transmission fiber at the signal wavelength,expressed in Nepers·km⁻¹ in place of more usual units dB/km: theattenuation in Nepers·km⁻¹ may be obtained by multiplying theattenuation expressed in dB/km by a factor log_(e)(10)/10. For thepurposes of the present invention, the effective length calculated withformula [8] may be approximated to: $\begin{matrix}{L_{eff} = \frac{1}{\alpha}} & \lbrack 9\rbrack\end{matrix}$as the exponential value at the numerator of formula [8] is close tozero for typical values of attenuation and span length.

In practice, the effective length calculated with formula [9] results tobe about 20 km for typical transmission fibers having an attenuationcoefficient of 0.2 dB/km. The effective length calculated with formulas[8] or [9] may be roughly used as a measure of the portion of fiber spanin which the power level of the optical signal reaches values that cancause nonlinearity to be detrimental for correct transmission. In otherwords, in a portion of fiber span downstream from the output of anoptical amplifier at a distance greater than an effective length one cansay that nonlinear effects do not play a substantial role, so that thedistortion of the signal in that span portion may be substantially dueonly to linear effects, such as chromatic dispersion.

It is known that the inclusion of an OPC 15 in an optical system mayreduce the negative effects produced on the optical signal bynonlinearity. The OPC device positioning has been related in the priorart to the compensation of the chromatic dispersion, so that the OPCdevice was at the mid-span point of the system, in proximity of theamplifier closer to the mid-span point. However, the Applicant has foundthat such positioning may not guarantee a sufficient reduction of theimpact of nonlinear effects in many cases, in particular for systemshaving high bit rate (e.g. Gbit/s) and/or long span lengths. Accordingto the Applicant, even if the positioning of the OPC device near themid-span point of the system may reduce nonlinearity, as the high powerregions are disposed roughly symmetrically with respect to the OPCdevice, the intrinsic asymmetry of the single high-power regions maystill cause high levels of penalty at the receiver. In particular thisproblem may arise with long average span lengths, i.e. in excess oftwo-three times the effective length, in which the power distributionalong each span has a great excursion between very high power values (atthe output of the amplifiers) and very low power values (at the end ofthe spans), i.e. more than about 3 dB below the maximum power level.

The Applicant has found that such problem may be solved by compensatingthe chromatic dispersion (or, more simply, dispersion) accumulated in aportion of the fiber span immediately upstream from the amplifier nearwhich the OPC device is disposed. The compensation is made immediatelyupstream such amplifier. The compensator used substantially compensatesthe dispersion accumulated by an optical signal traveling along theoptical fiber path in a portion of span having a length of(L_(amp)−L_(eff)). For the purposes of the present invention, asubstantial compensation occurs when the dispersion accumulated alongthe optical path of length (L_(amp)−L_(eff)) is compensated at a levelbetween 85% and 115%. Preferably, compensation may occur at a level ofat least 90%. Preferably, compensation may occur at a level of at most110%.

As defined above, for the purposes of the present invention a dispersioncompensator is a device shorter than the length of span portion whosedispersion it compensates. Preferably the length of the dispersioncompensator is shorter than ⅓ of the length of the span portion whosedispersion it compensates. The dispersion compensator may be an opticalfiber having a dispersion coefficient D greater in absolute value thanthe D of the span portion whose dispersion it compensates, preferablygreater by at least three times. For example, it can be a dispersioncompensating fiber, or even a transmission fiber with a greater D thanthe D of the fibers included in the span portion whose dispersion iscompensated. The dispersion compensator may even be much shorter thatthe span portion, for example it can be fiber grating dispersioncompensator based on a chirped fiber grating, with a length in the rangefrom tens of centimeters to few meters, or an optical waveguide device,a micro optics device, or another compact device. In general, therelatively short length and/or low attenuation of the dispersioncompensator of the invention achieves the advantage of limiting theonset of additional nonlinear effects, even if an amplifier is combinedwith the compensator to recover the compensator loss.

According to a first preferred embodiment, a dispersion compensator 16,e.g. a length of dispersion compensating optical fiber, having a sign ofthe dispersion opposite with respect to the sign of the dispersioncoefficient of the transmission fiber included in the fiber spans isadded. In alternative to a dispersion compensating optical fiber, adifferent type of compensator can be used, such as for example adispersion compensating grating. FIG. 4 is a schematic enlargement ofthe portion of optical line including the OPC device 15 of the system 10in FIG. 1. As it can be seen, the compensator 16 is arranged upstreamfrom an optical amplifier 13 ⁴ disposed along the optical line. The OPCdevice is connected to the optical amplifier 13 ⁴. The compensator 16 isconnected at a first end 17 to the output of the span 14 ³ immediatelyupstream from the amplifier 13 ⁴ and at a second end 18 to the OPCdevice 15. In an alternative configuration, not shown, the OPC device 15may be connected downstream from the optical amplifier 13 ⁴, so that thecompensator is connected between the output end of the span 14 ³ and theinput end of the optical amplifier 13 ⁴.

FIG. 5 schematically shows the corresponding behavior of the opticalpower and of the chromatic dispersion of a signal traveling in theportion of optical line shown in FIG. 4. For example, it may be assumedthat the sign of dispersion of the transmission fiber included in thespans of the optical line is positive at the signal wavelength, so thatthe sign of dispersion of compensator 16 is negative. The behavior ofthe optical power of the optical signal is shown in FIG. 5 by thecontinuous line. As it can be seen, the optical power increases up to amaximum level at the amplifier 13 ³ and then decreases due to theattenuation of the fiber included in the span 14 ³. After a portion ofspan having a length L_(eff), the optical power has decreased until alevel at which it may be supposed that nonlinear effects do not play asubstantial role, so that the optical system behaves practicallylinearly. At the end of the span, that is, after a length L_(amp), thesignal passes through the compensator 16 and the OPC device 15, and thenis re-amplified by amplifier 13 ⁴, so that the optical power increasesabruptly to the maximum level, and so on. Advantageously, the opticalpower is already sufficiently low when the optical signal enters in thecompensator 16, so that nonlinearity is not added by the presence of thecompensator 16. On the other hand, the behavior of accumulateddispersion is shown in FIG. 5 by the dashed line. As it can be seen, theaccumulated dispersion grows substantially linearly along the span 14 ³,starting from an initial value which practically depends on the distanceof the considered span from the insertion point of the signal in theoptical line. If D_(f) is the dispersion coefficient at the signalwavelength of the transmission optical fiber included in the fiber span14 ³, the total accumulated dispersion in the span is D_(f)·L_(amp). Thecompensator 16 has a length L_(comp) suitable for compensating a portionof the dispersion accumulated in the fiber span 14 ³. More particularly,as it can be seen in FIG. 5, the length L_(comp) is chosen so that thecompensator 16 substantially compensates for the accumulated dispersionin a portion of span having a length (L_(amp)−L_(eff)). After thepassage through the compensator 16, the optical signal enters in the OPCdevice 15, for being subjected to phase conjugation (arrow in FIG. 5).At the output of the OPC device 15, the sign of the accumulateddispersion of the optical signal is changed, whereas its absolute valueremains substantially unchanged. Downstream from the OPC device, theaccumulated dispersion diminishes in absolute value, leading towardsdispersion compensation at a certain point along the system, typicallynear the end of the optical line. Due to the added compensator 16, aresidual dispersion may remain uncompensated at the end of the opticalsystem. A suitable additional compensator may be provided at the end ofthe system in order to compensate such residual dispersion.

The Applicant has found that a configuration according to the firstembodiment above described with reference to FIGS. 4 and 5 allows toreduce the impact of nonlinearity. In a third simulation performed bythe Applicant, an optical signal was sent in a system including sixspans of length L_(amp)=100 km of a fiber having an attenuationcoefficient of 0.25 dB/km (L_(eff)≅17 km), a nonlinear coefficient of1.3 1/(W·km) and a dispersion coefficient, expressed as a group velocitydispersion, of ˜20 ps²/km. At the input of each span, the optical powerof the optical signal was set to 10 dBm. An ideal OPC device, performingonly phase conjugation, was disposed at the end of the third span,before an amplifier. Different lengths of a dispersion compensatingfiber having a dispersion coefficient, expressed as group velocitydispersion, of +80 ps²/km were added at the end of the third span,upstream from the OPC device. For each length of dispersion compensatingfiber considered, the eye opening penalty (EOP) was evaluated at the endof the system. In order to correctly evaluate the penalty, the chromaticdispersion not compensated for by the OPC device due to the added lengthof compensating fiber was supposed to be separately compensated at thereceiver. The result of the simulation is reported in FIG. 6, that showsthe EOP versus the added length of dispersion compensating fiberL_(comp). As it can be seen, with no added piece of dispersioncompensating fiber (L_(comp)=0), a penalty of about 1 dB is found. Witha length around 20 km of dispersion compensating fiber, penalty levelslower than 0.4 dB may be found, that is, more than 0.5 dB better thanthe previous case. Such length of dispersion compensating fibercorresponds to the compensation of the accumulated dispersion in 80 kmof the transmission fiber (1600 ps²), that is, approximately(L_(amp)−L_(eff)). The use of a dispersion compensating fiber having adispersion coefficient higher than the dispersion coefficient of thetransmission fiber (in absolute value) advantageously allows to add apiece of fiber having a lower length with respect to (L_(amp)−L_(eff)),so that the additional attenuation introduced by the compensating fibermay be very low. In order to keep low such additional attenuation, thedispersion coefficient of the dispersion compensating fiber may bepreferably three times or more higher than the dispersion coefficient ofthe transmission fibers, in absolute value, at the signal wavelength.

According to the Applicant, the reduction of the impact of nonlinearityshown by the above simulation may depend from the fact that asubstantially symmetric disposition of the high power regions withrespect to the accumulated dispersion is obtained when adding thecompensator according to what stated above.

FIG. 7 a and FIG. 7 b schematically show plots of the optical power ofan optical signal which can be obtained by propagating the same along anoptical line comprising four spans of optical fiber and four amplifiers,versus the dispersion accumulated by the same optical signal. In bothfigures, it is supposed that an OPC device is placed before the thirdamplifier. In FIG. 7 a, it is supposed that no additional compensator ispresent upstream from the OPC device, whereas in FIG. 7 b it is supposedthat an additional compensator is present between the end of the secondspan and the OPC device. The compensator compensates for the accumulateddispersion in a portion of span having a length (L_(apm)−L_(eff)). Inboth figures, high power regions having a length L_(eff) arehighlighted.

Considering FIG. 7 a first, at the input of the system the dispersionaccumulated by an optical signal is zero (or at a predetermined value ifpre-chirp is used) and the first amplifier (AMP#1) sets the opticalpower of the optical signal to a predetermined high level. During travelon the first span the signal accumulates an amount of dispersion (DaccSP #1), in dependence of the dispersion coefficient of the fiber used,while at the same time the optical power diminishes due to fiberattenuation. At the end of the first span the optical signal isamplified by the second amplifier (AMP #2), that substantially bringsthe optical power up to the same level set by AMP #1. During travel onthe second span, the signal continues to accumulate dispersion (Dacc SP#2), while the power diminishes, up to the OPC device. The OPC deviceperforms optical phase conjugation, so that the accumulated dispersionat the end of the second span is folded on the opposite side of thegraph, substantially at a symmetric position. At the output of the OPCdevice, the phase conjugated optical signal is amplified by the thirdamplifier (AMP #3), that substantially brings the optical power up tothe same level set by AMP #1 and/or AMP #2. During travel on the thirdspan, the phase conjugated signal reduces its accumulated dispersion(Dacc SP #3), in absolute value, while the power diminishes. Then thephase conjugated optical signal is amplified by the fourth amplifier(AMP #4) and transmitted to the fourth span, where it reduces itsaccumulated dispersion down to substantially zero at the end of thesystem. As it can be seen in FIG. 7 a, the highlighted high powerregions are not symmetric with respect to the zero value of accumulateddispersion.

In FIG. 7 b, the high-power regions upstream and downstream from the OPCdevice have been staggered for better clarity. In the case shown in FIG.7 b, in the first two spans the system behaves in the same way as forthe case shown in FIG. 7 a.

However, this time, at the end of the second span a compensator (COMP)brings the accumulated dispersion at the same level that the opticalsignal had after traveling the first portion of the second span having alength approximately equal to L_(eff). In other words, the compensatoradded at the end of the second span compensates for the accumulateddispersion in the linear portion of the second span, having a length(L_(amp)−L_(eff)). Then the OPC device performs phase conjugation on theoptical signal, changing the sign of accumulated dispersion close to thethird amplifier (AMP #3). During travel on the third span, the phaseconjugated signal reduces its accumulated dispersion (Dacc SP #3), inabsolute value, while the power diminishes. Then the phase conjugatedoptical signal is amplified by the fourth amplifier (AMP #4) andtransmitted to the fourth span, where it reduces its accumulateddispersion down to substantially zero at the end of the last high-powerregion. After that, the optical signal accumulates a residual dispersionup to the end of the system, in a portion of the fourth span in whichthe power level is low, so that the system behaves linearly. Thus, suchresidual accumulated dispersion may be compensated for (not shown forsimplicity in FIG. 7 b) by a further compensator placed at the end ofthe fourth span. As it can be seen in FIG. 7 b, this time the high-powerregions are disposed substantially symmetrically with respect to thezero point of accumulated dispersion. According to the results obtainedby the Applicant, this is of benefit for reducing nonlinearity, at leastin a similar measure to the benefit obtained by disposing the high-powerregions symmetrically in space with respect to the positioning of theOPC device.

According to a second preferred embodiment, a compensator, e.g. a lengthof optical fiber, having a dispersion with the same sign as the sign ofthe dispersion coefficient of the transmission fiber included in thefiber spans is added. In such case, the OPC device is disposed upstreamfrom the compensator. FIG. 8 is a schematic enlargement of the portionof optical line including the OPC device 15 of the system 10 in FIG. 1.The same reference numbers of FIG. 4 are used for indicating similarcomponents. As it can be seen, a length of additional optical fiber 16is disposed between the OPC device 15 and an optical amplifier 13 ⁴disposed along the optical line, so that it is connected to the OPCdevice 15 at a first end 17 and to the optical amplifier 13 ⁴ at asecond end 18 thereof. In alternative to a length of additional opticalfiber, a different type of compensator can be used, such as for examplea grating. The additional optical fiber 16 is suitable for introducingan accumulated dispersion substantially equal to the dispersionaccumulated in a portion of the span 14 ³ having a length(L_(amp)−L_(eff)).

FIG. 9 schematically shows the corresponding behavior of the opticalpower and of the chromatic dispersion of a signal traveling in theportion of optical line shown in FIG. 8. For example, it may be assumedthat the sign of dispersion of the transmission fiber included in thespans of the optical line is positive at the signal wavelength, so thatthe sign of dispersion of compensator 16 is equally positive. Thebehavior of the optical power of the optical signal is shown in FIG. 9by the continuous line. As it can be seen, the optical power increasesup to a maximum level at the amplifier 13 ³ and then decreases due tothe attenuation of the fiber included in the span 14 ³. After a portionof span having a length L_(eff), the optical power has decreased until alevel at which it may be supposed that nonlinear effects do not play asubstantial role, so that the optical system behaves practicallylinearly. At the end of the span, that is, after a length L_(amp), thesignal passes through the OPC device 15 and the compensator 16, and thenis re-amplified by amplifier 13 ⁴, so that the optical power increasesabruptly to the maximum level, and so on. Advantageously, the opticalpower of the phase conjugated signal in output from the OPC device 15may be kept sufficiently low, so that when the phase conjugated opticalsignal enters in the compensator 16 nonlinearity does not substantiallyarise. On the other hand, the behavior of accumulated dispersion isshown in FIG. 9 by the dashed line. As it can be seen, the accumulateddispersion grows substantially linearly along the span 14 ³, startingfrom an initial value which practically depends on the distance of theconsidered span from the insertion point of the signal in the opticalline. If D_(f) is the dispersion coefficient at the signal wavelength ofthe transmission optical fiber included in the fiber span 14 ³, thetotal accumulated dispersion in the span is D_(f)·L_(amp). Then theoptical signal enters in the OPC device 15, for being subjected to phaseconjugation (arrow in FIG. 9). At the output of the OPC device 15, thesign of the accumulated dispersion of the optical signal is changed,whereas its absolute value remains substantially unchanged. Thecompensator 16 has a length L_(comp) suitable for introducing anaccumulated dispersion equal to the dispersion accumulated in a portionof the fiber span 14 ³. More particularly, the length L_(comp) is chosenso that the compensator 16 introduces an accumulated dispersionsubstantially equal to the dispersion accumulated in a portion of spanhaving a length (L_(amp)−L_(eff)). Due to phase conjugation, theaccumulated dispersion diminishes in absolute value in the compensator16. Then, the accumulated dispersion continues its decrease, leadingtowards dispersion compensation at a certain point along the system,typically near the end of the optical line. Due to the added compensator16, a residual dispersion may remain uncompensated at the end of theoptical system. A suitable additional compensator may be provided at theend of the system in order to compensate such residual dispersion.

Also in this case, if an additional piece of optical fiber is used ascompensator, it is preferred to use an additional fiber having adispersion coefficient higher than the dispersion coefficient of thetransmission fibers used in the spans of the system. This advantageouslyallows to add a piece of fiber having a lower length with respect to(L_(amp)−L_(eff)), so that the additional attenuation introduced by theadditional fiber may be very low. In order to keep low such additionalattenuation, the dispersion coefficient of the additional fiber may bepreferably three times or more higher than the dispersion coefficient ofthe transmission fibers, at the signal wavelength.

FIG. 10 shows a diagram of the optical power versus the accumulateddispersion similar to those shown in FIGS. 7 a and 7 b, for a systemhaving a compensator and an OPC device arranged according to the secondembodiment. In FIG. 10, the high-power regions upstream and downstreamfrom the OPC device have been staggered for better clarity. In the caseshown in FIG. 10, in the first two spans the system behaves in the sameway as in the case shown in FIG. 7 a. At the end of the second span, theOPC device performs optical phase conjugation, so that the accumulateddispersion is folded on the opposite side of the plot, substantially ata symmetric position. Then, before the third amplifier (AMP #3), acompensator (COMP) introduces a quantity of accumulated dispersionsubstantially equal to the dispersion accumulated in the linear portionof the second span, having a length (L_(amp)−L_(eff)), so that the phaseconjugated signal reduces its accumulated dispersion, in absolute value,of the same quantity. Then the third amplifier (AMP #3) amplifies thephase conjugated signal. During travel on the third span, the phaseconjugated signal still reduces its accumulated dispersion (Dacc SP #3),in absolute value, while the power diminishes. The phase conjugatedoptical signal is finally amplified by the fourth amplifier (AMP #4) andtransmitted to the fourth span, where it reduces its accumulateddispersion down to substantially zero at the end of the last high-powerregion. After that, the optical signal accumulates a residual dispersionup to the end of the system, in a portion of the fourth span in whichthe power level is low, so that the system behaves linearly. Thus, suchresidual accumulated dispersion may be compensated for (not shown forsimplicity in FIG. 10) by a further compensator placed at the end of thefourth span. As it can be seen in FIG. 10, the high-power regions aredisposed substantially symmetrically with respect to the zero point ofaccumulated dispersion.

Preferably, the OPC device may be disposed in proximity of the mid-spanoptical amplifier. If the optical system has N spans between its inputand its output, the mid-span optical amplifier is the [N/2+1]^(th) (tobe understood as the integer part of N/2+1) optical amplifier, startingthe counting of the optical amplifiers from the input of the opticalfiber path. This particular positioning is preferred in that it allowsat the same time to reduce in a very effective manner the effects ofnonlinearities and to compensate chromatic dispersion to a great extent,except for a residual chromatic dispersion that may be compensatedseparately, for example at the end of the optical fiber path. Further,the reduction of the effects of nonlinearities may be very effectivewith a positioning near the mid-span, as in this case the high-powerregions will be disposed symmetrically with respect to the OPC. However,the Applicant has found that positive effects in the reduction of theimpact of nonlinearity may be obtained by positioning the OPC near anamplifier disposed within a mid-span portion of the optical fiber pathof ±L/5, preferably ±L/6, around the mid-span point of the optical fiberpath, wherein L is the overall length of the optical fiber path. Anyway,it has to remembered that if the positioning of the OPC device is madeaway from the mid-span optical amplifier, then a substantial amount ofchromatic dispersion not compensated by the OPC device needs to becompensated. This may be done once at the end of the optical fiber path,preferably with one or more compensating gratings, or more graduallyalong the optical fiber path with suitable compensating devices, forexample included in at least some optical amplifier, provided that thesymmetry in the distribution of the high power regions along the opticalpath of the system with respect to accumulated dispersion is preserved.

FIG. 11 shows the result of an experiment performed by the Applicantwith a configuration according to FIG. 4. A NRZ signal having a bit rateof 10 Gbit/s, an average power of 12 dBm and a central wavelength of1552.5 nm was launched over an optical line made of four spans ofFreeLight™ fiber, produced by FOS (Italy), having a group velocitydispersion of about −5 ps²/km at 1550 nm and an attenuation of about 0.2dB/km at 1550 nm (that is, L_(eff)≅21 km). The spans had an averagelength L_(amp) of about 100 km (the actual lengths were between 99 kmand 104 km), so that (L_(amp)−L_(eff))≅79 km. The high average power ofthe pulse launched in the experimental system was chosen in order tocause strong nonlinear effects to occur, in view of the relatively shortlength of the overall system (about 400 km). An OPC device was disposedbetween the input of the third amplifier and the output end of a pieceabout 4.9 km long of dispersion compensating optical fiber having agroup velocity dispersion of +80 ps²/km, connected at the end of thesecond span, so as to compensate dispersion accumulated in about 78 kmof FreeLight™ fiber. The OPC device was realized by use of twosemiconductor optical amplifiers (SOAs) disposed in a Mach-Zehnderconfiguration between two polarization beam splitters, according to apolarization diversity scheme, to achieve polarization independentoperation. Each of the SOA along the arms of the Mach-Zehnder was anOptospeed SOA1550MRI/X with a guide length of 1.5 mm. Optical phaseconjugation was achieved in each SOA by four-wave-mixing. A pump sourcewith a wavelength of 1550.9 nm was used to this end. The outputwavelength of the phase conjugated pulse was 1549.3 nm. In FIG. 11, thethick continuous line shows the spectrum of the pulse after propagationat the end of the system. The x-axis is normalized versus the centralwavelength of the received phase conjugated signal, whereas the y-axisis normalized versus the peak power of the received pulse.

The experiment was then repeated by eliminating the piece of dispersioncompensating fiber, leaving only the OPC device. The thin continuousline in FIG. 11 shows the spectrum of the received pulse, normalizedversus central wavelength and peak power.

The experiment was further repeated by eliminating also the OPC device.The thin dashed line in FIG. 11 shows the spectrum of the receivedpulse, normalized versus central wavelength (this time unchanged duringpropagation) and peak power.

As it can be seen, the spectrum of the pulse in the system without theOPC device is quite large, due to the onset of nonlinear effects in theoptical line (mainly self-phase-modulation). The reduction of the effectof nonlinearity with the configuration having the OPC and the additionaldispersion compensating fiber is clearly visible. FIG. 11 also shows asubstantial reduction versus a configuration having only the OPC device.

FIG. 12 shows the results of a fourth simulation performed by Applicantunder conditions corresponding to those of the now described experiment,with the only variation that the bit rate of the NRZ signal was of 40Gbit/s in the simulation. For the fourth simulation, different lengthsof dispersion compensating fiber were introduced and the eye openingpenalty (EOP) at the receiver was evaluated for each added length(compensating fiber added in steps of 200 m). The residual dispersioncaused by the addition of the piece of dispersion compensating fiber waslinearly compensated at the end of the system. The line in FIG. 12 showsthe penalty versus the length L_(comp) of added dispersion compensatingfiber. As it can be seen, a minimum of penalty is found for L_(comp) ofabout 4.9 km, showing a good matching with the results of theexperiment.

FIG. 13 a-b show the eye diagram of the pulses of the previous fourthsimulation, in a system including 4.9 km of additional fiber having adispersion of +80 ps²/km and, respectively, of the pulses at the outputof a system having an OPC but no additional fiber (point L_(comp)=0 inFIG. 12). FIG. 13 a corresponds to a penalty of 0.73 dB, whereas FIG. 13b corresponds to a complete eye closure (very high penalty).

FIG. 14 shows the results of a fifth simulation performed by theApplicant. A RZ signal, having a gaussian shape, full-width athalf-maximum duration T_(FWHM) of 5 ps, average power 10 dBm, modulatedat 40 Gbit/s by a PRBS word having a length of 32 bit, was launched overa system having eighteen spans of fiber. The spans had a length of 100km. The fiber had a group velocity dispersion of −20 ps²/km, anattenuation of 0.2.dB/km and a nonlinear coefficient of 1.3 1/(W·km). Atthe beginning of each span lumped (EDFA) amplification was introduced.An ideal OPC device, performing only phase conjugation, was introducedat the end of the ninth span, before the tenth amplifier. A piece ofdispersion compensating fiber having a group velocity dispersion of +80ps²/km was added between the input of the OPC device and the output endof the ninth span, according to the configuration of FIG. 4. A nonlinearcoefficient of zero was set for the dispersion compensating fiber, assuch fiber is placed in a substantially linear region, so that thesimulation result would not change to a significant extent even if thenonlinearity of the dispersion compensating fiber was considered. Forthe simulation, different lengths of dispersion compensating fiber wereintroduced and the eye opening penalty (EOP) at the receiver wasevaluated for each added length (compensating fiber added in steps of200 m). The residual dispersion caused by the addition of the piece ofdispersion compensating fiber was linearly compensated at the end of asystem. The line in FIG. 14 shows the penalty versus the length L_(comp)of added dispersion compensating fiber. As it can be seen, a minimum ofpenalty in a region around 20 km, between about 16 km and 22 km isfound. Such minimum may reach values of penalty lower than the penaltyof a system not including the additional fiber (see point L_(comp)=0 inFIG. 14) by more than 2 dB.

FIG. 15 a-b show the eye diagrams of the pulses of the previous fifthsimulation, respectively at the output of a system including 20 km ofadditional fiber having opposite sign of dispersion with respect to thefiber used in the spans (FIG. 15 a) and of a system (FIG. 15 b) havingan OPC but no additional fiber (point L_(comp)=0 in FIG. 14). FIG. 15 acorresponds to a penalty of 0.80 dB, whereas FIG. 15 b corresponds to apenalty of 2.92 dB.

In a sixth simulation, the added compensating fiber had a group velocitydispersion of −80 ps²/km. The OPC device was introduced at the outputend of the ninth span and the additional fiber was connected between theoutput of the OPC device and the input of the tenth amplifier, accordingto the configuration of FIG. 8. The rest of the parameters consideredfor this sixth simulation were the same of the fifth simulation. Thissixth simulation was performed in a similar way to the previous one. Theresult is identical to the line resulting in FIG. 14 from the previoussimulation.

In a seventh simulation, carried out for a system according to the fifthsimulation, the OPC device and the additional fiber were put upstreamfrom amplifiers different from the tenth one (that is, different fromthe mid-span amplifier). The length of the additional fiber was 20 km.The residual dispersion was supposed to be compensated once at the endof the system. FIG. 16 shows the result of the simulation, as a plot ofEOP versus the amplifier number (AMP #). As it can be seen, a minimum ofpenalty may be found for the mid-span amplifier (AMP #10): however, thepenalty does not increase a lot in a region between the eighth and thefourteenth amplifier (that is, between 700 km and 1300 km). Thus, thesystem may tolerate a displacement of the OPC device with respect to themid-span amplifier, if the positioning of the OPC at the mid-spanamplifier is not convenient for practical installation reasons.

The system according to the invention has been explained with referenceto an optical fiber path included between a transmitting station and areceiving station.

This has not to be considered as limiting the invention, as an opticalline including an optical fiber path according to what stated above maybe disposed in a more complex network between any two nodes the networkitself, for example two nodes of an optical network not havingtransmitting and/or receiving function, but only routing function.

The system or the optical line according to the invention may beimplemented ex-novo, by connecting at least the various componentsdescribed with reference to FIG. 1 and FIG. 4 or FIG. 8 and preferablyproviding that the compensator 16 and the OPC device 15 be included inthe same amplification site of the associated amplifier. Lesspreferably, the compensator 16 and the OPC device 15 may be included ina separated site.

The system or the optical line according to the invention may further bean upgrade of an already installed system. In such case, it may bepossible to provide the OPC device 15 and the compensator 16 arrangedaccording to the invention, so as to include both OPC device 15 andcompensator 16 in the same amplification site of the associatedamplifier. Less preferably, the OPC device 15 and the compensator 16 maybe included in a separated site.

1-11. (canceled)
 12. An optical system comprising: an optical fiber pathsuitable for propagating an optical signal at least in a firstdirection; a plurality M of optical amplifiers disposed along saidoptical fiber path so as to divide said optical fiber path in N spans ofoptical fiber, said spans of optical fiber having substantially a lengthL_(amp) and comprising at least one transmission optical fiber having aneffective length L_(eff), and an optical phase conjugation deviceassociated to an amplifier of said plurality of amplifiers, said opticalphase conjugation device being disposed in combination with a dispersioncompensator, said compensator being disposed upstream from saidamplifier associated to the optical phase conjugated device, saidcompensator being adapted for introducing an accumulated dispersion suchas to substantially compensate a dispersion accumulated in a portionhaving a length (L_(amp)−L_(eff)) of a span immediately upstream fromsaid amplifier associated to said optical phase conjugation device. 13.The optical system according to claim 12, wherein said dispersioncompensator has a sign of dispersion opposite to a sign of dispersion ofsaid span immediately upstream from said amplifier at a wavelength ofsaid optical signal, and said optical phase conjugation device isdisposed downstream from said dispersion compensator.
 14. The opticalsystem according to claim 12, wherein said dispersion compensator has asign of dispersion equal to a sign of dispersion of said spanimmediately upstream from said amplifier at a wavelength of said opticalsignal, and said optical phase conjugation device is disposed upstreamfrom said dispersion compensator.
 15. The optical system according toclaim 12, wherein said dispersion compensator includes a length ofoptical fiber.
 16. The optical system according to claim 15, whereinsaid length of optical fiber has an absolute value of dispersioncoefficient higher than or equal to three times the dispersioncoefficient of said transmission optical fiber at a wavelength of saidoptical signal.
 17. The optical system according to claim 12, whereinsaid dispersion compensator further comprises a chirped fiber grating.18. The optical system according to claim 13, wherein said dispersioncompensator further comprises a chirped fiber grating.
 19. The opticalsystem according to claim 14, wherein said dispersion compensatorfurther comprises a chirped fiber grating.
 20. The optical systemaccording to claim 12, wherein said optical amplifiers compriseerbium-doped fiber amplifiers.
 21. The optical system according to claim12, further comprising a transmitting station and a receiving station,said transmitting station being connected at an input end and saidreceiving station being connected to an output end of said optical fiberpath.
 22. A method for assembling an optical system comprising the stepsof: providing a plurality M of optical amplifiers; connecting saidplurality of optical amplifiers by N spans of optical fiber so as toform an optical fiber path, said spans of optical fiber havingsubstantially a length L_(amp) and comprising at least one transmissionoptical fiber having an effective length L_(eff); and associating aphase conjugation device to an amplifier along said optical fiber path;said step of associating comprising: disposing a compensator upstreamfrom said amplifier associated to the optical phase conjugated device,and disposing said phase conjugation device in combination with saiddispersion compensator, said compensator being adapted for introducingan accumulated dispersion such as to substantially compensate adispersion accumulated in a portion having a length (L_(amp)−L_(eff)) ofa span immediately upstream from said amplifier associated to saidoptical phase conjugation device.
 23. A method of operating of anoptical transmission system comprising an optical fiber path comprisingat least one transmission optical fiber having an effective lengthL_(eff) and a plurality of optical amplifiers disposed along saidoptical fiber path so as to divide said optical fiber path in N spans ofoptical fiber having substantially a length L_(amp), comprising:inserting an optical signal at an input end of said optical fiber path;amplifying said optical signal along said fiber spans by said pluralityof optical amplifiers; accumulating a dispersion of said optical signalalong said optical fiber path; phase-conjugating said optical signalnear a first amplifier of said plurality of optical amplifiers so thatsaid optical signal diminishes its accumulated dispersion, in absolutevalue, after said step of phase-conjugating; and passing said opticalsignal, before said first amplifier, through a compensator, saidcompensator introducing an accumulated dispersion such as tosubstantially compensate a dispersion accumulated in a portion having alength (L_(amp)−L_(eff)) of a span immediately upstream from said firstamplifier.
 24. A method of upgrading an optical transmission systemcomprising an optical fiber path, the optical fiber path including atleast one transmission optical fiber having an effective length L_(eff)and a plurality of optical amplifiers disposed along said optical fiberpath so as to divide said optical fiber path in N spans of optical fiberhaving substantially a length L_(amp), comprising: disposing a phaseconjugation device in association with one of said plurality of opticalamplifiers in combination with a dispersion compensator disposedupstream from said one amplifier, said compensator being adapted forintroducing an accumulated dispersion such as to substantiallycompensate a dispersion accumulated in a portion having a length(L_(amp)−L_(eff)) of a span immediately upstream from said one amplifierassociated to said optical phase conjugation device.